1. Field of the Invention
The present invention relates to a space diversity (referred to hereinafter as SD) reception system of a radio signal. More particularly, this invention relates a SD reception system which is simple in its configuration and requires less severe specifications for its automatically gain-controlled intermediate frequency amplifier (referred to hereinafter as IF AGC amplifier).
2. Description of the Related Art
For receiving a radio signal of a digital radio equipment, etc., SD reception systems have been widely employed, where outputs of a first receiver for receiving a signal from a first antenna and a second receiver for receiving a signal from a second antenna are combined to attain a stable receiving signal even when the conditions of the received signals by the antennas are fluctuating due to a fading or multipath effect, etc., so that the signal is reliably received.
FIG. 1 shows a prior art SD reception system. The numerals 1 and 11 respectively denote microwave low-noise pre-amplifiers (MFA); the numerals 2 and 12 respectively denote microwave variable attenuators (MVA); the numerals 3 and 13 respectively denote frequency converters (FC); the numerals 4 and 14 respectively denote intermediate-frequency amplifiers (referred to hereinafter as IF amplifiers) (IFA); the numerals 5 and 15 respectively denote automatically gain-controlled (referred to hereinafter as AGC) amplifiers; the numerals 6 and 16 respectively denote amplitude detectors (DET); the numerals 7 and 17 respectively denote direct-current (DC) amplifiers (DCA); the numerals 8 and 18 respectively denote level monitor detectors (DET); the numeral 9 and 19 respectively denote band-pass filters (BPF) of the IF band; the numeral 21 denotes a microwave local frequency oscillator (LO); the numeral 22 denotes a microwave hybrid junction (H); the numeral 23 denotes a microwave endless Phase shifter (EPS); the numeral 24 denotes a phase control circuit (PHC); the numeral 25 denotes a phase comparator (PC) of the IF band; the numeral 26 denotes a 90.degree. phase shifter (PS) of the IF band; the numerals 27.sub..about. 29 respectively denote DC amplifiers; the numerals 30 and 31 respectively denote variable attenuators (IVA) of the IF band; the numeral 32 denotes a hybrid junction (H) of the IF band; the numeral 33 denotes a third AGC amplifier of the IF band; the numeral 34 denotes an amplitude detector (DET); and the numeral 35 denotes a DC amplifier (DCA).
Microwave signals received by a first antenna ANT 1 and a second antenna ANT 2 are amplified by pre-amplifiers 1 and 11, respectively; the output signal levels therefrom are adjusted by variable attenuators 2 and 12; the output signals therefrom are converted into IF signals by frequency converters 3 and 13 with the local oscillator frequency; and the IF signals are amplified by IF amplifiers 4 and 14, which are of low-noise amplifiers. The IF signals output from IF amplifiers 4 and 14 are respectively amplified by AGC amplifiers 5 and 15, where detectors 6 and 16 output DC signals varying in accordance with the output levels of AGC amplifiers so as to form feedback loops via DC amplifiers 7 and 17 to variable attenuators 2 and 12 as well as AGC IF amplifiers 5 and 15; accordingly, outputs of AGC amplifiers 5 and 15 are kept constant even when the input levels thereto are fluctuated. Monitoring of the levels of the received signals are individually carried out by observing each of the AGC voltages, or by detectors 8 and 18 detecting the signal levels at inter-stage of AGC amplifiers 5 and 15.
Each of the IF signals is co-phased with each other as follows. Outputs of AGC IF amplifiers 5 and 15 are input to narrow-band bandpass filters 9 and 19 so as to allow carrier components of the IF signals input thereto to pass. Output of bandpass filter 9 is input directly to phase comparator 25, while output of another bandpass filter 19 is input via a 90.degree. phase shifter 26 to phase comparator 26, where the phases of the IF signals input thereto are compared so as to output the phase difference therebetween. Thus detected phase difference is applied via phase controller 24 to endless phase shifter 23 so as to adjust phase of the local oscillator signal to be input from hybrid junction 22 to frequency converter 13, while output of local oscillator 21 is directly input from hybrid junction 22 to frequency converter 3, so that the phases of the two IF signals output from two AGC IF amplifiers 5 and 15 are always kept in phase with each other.
Furthermore, on the other hand, the outputs of AGC IF amplifiers 5 and 15 to be combined by hybrid junction 32 are attenuated by first and second variable attenuator 30 and 31, respectively, to adjust the signal levels as described below. Differential amplifier 27 detects the difference of the AGC voltages respectively generated for two AGC IF amplifiers 5 and 15; and the detected difference is amplified by DC amplifiers 28 and 29, one of which outputs an opposite polarity signal of the other. Outputs of DC amplifiers 28 and 29 control attenuation of first and second variable attenuators 30 and 31, respectively, so that the IF signal levels input to hybrid junction 32 are corresponding to the microwave signal levels received by the first and second antennas, ANT 1 and ANT 2, respectively. In other words, when the microwave signal level received by first antenna ANT 1 is higher than that of the second antenna ANT 2, the attenuation by first variable attenuator 30 is adjusted to be less than that of second variable attenuator 31. This is because, if the outputs of the same level from two AGC IF amplifiers 5 and 15 are combined by hybrid junction 32, the distortion or low SIN ratio carried in the signal of the lower level is included in the combined signal by fifty-fifty share. Therefore, the inputs to the hybrid junction 32 must be adjusted so that the inputs to the hybrid junction follow the input levels of respective antennas.
The combined IF signal output from hybrid junction 32 is amplified by third AGC IF amplifier 33. Detector 34 detects signal level of third AGC IF amplifier 33 so as to output a DC signal varying in accordance with the output signal level of third AGC IF amplifier 33. The output DC signal is fed back via a DC amplifier 35 to third AGC IF amplifier 33, so that the output level of third AGC IF amplifier 33 is stabilized constant.
In the prior art SD reception system shown in FIG. 1, there are the following problems. That is, as many as three AGC IF amplifiers 5, 15 and 33 are required; furthermore, the specifications required in these amplifiers are severe because they are employed in the main signal route, where linearity characteristics, phase characteristics as well as saturation characteristics are strictly required. Moreover, the IF signals once amplified by AGC IF amplifiers 5 and 15 must be attenuated at the expense of the additional variable attenuators 30 and 31 and control circuits 27.sub..about. 29 therefor.